Most of international oilfields employ sucker-rod pumping equipment. A sucker rod drives a sucker pump to reciprocate up and down in a tubing, so as to lift oil to the ground, and transport it to a superior-level system through a pipeline. A wellhead sealing device is mounted above a wellhead oil pipeline. In general, the wellhead sealing device includes a blowout prevention unit, a closing unit, a declination rectification unit and a packing box, and a core part of its operation is the packing box, and a wellhead of the sucker-rod pumping machine in an operating state is sealed by compacting a packing in the packing box. A function of the wellhead sealing device is to ensure that the wellhead is sealed when the sucker-rod pumping machine is in an operating state, a packing replacing state and a rod breaking state.
A sealing mechanism of the wellhead sealing device of the sucker-rod pumping machine is that the packing is filled in an annular space formed by a polished rod and the packing box; and a gland extrudes the packing downwards to deform it, so as to increase pressure intensities among the polished rod and the inner wall of the packing box and the packing, so that the purpose of sealing the wellhead is achieved.
The wellhead sealing device of the sucker-rod pumping machine mainly includes the following units:
a blowout prevention unit, which does not affect the normal operation of the sucker-rod pumping machine daily, and when the polished rod of the sucker-rod pumping machine is broken off, an action component of the blowout prevention unit will rapidly automatically plug the wellhead under an impact of a liquid;
a closing unit, which does not affect the normal operation of the sucker-rod pumping machine daily, and when the sucker-rod pumping machine is shut down and the packing in the packing box is replaced, the closing unit is manually operated, so that the packing in the packing box is extruded to deform and close the wellhead;
a declination rectification unit, which is operated under a closing state, so that the packing and the polished rod are statically coaxial to achieve declination rectification of the packing box;
and
a packing box, which is used for placing a plurality of packings therein and deforms by extruding the packing downwards, so as to seal the wellhead when the sucker-rod pumping machine is in the operating state. In an ideal state, the packing box must be kept coaxial with the polished rod (here the packing box or a packing group is kept in parallel and concentrical with the polished rod constantly, which is called dynamic coaxial), and meanwhile, sufficient lubrication between the packing and the polished rod is ensured, and eccentric wear and dry grinding of the packing are avoided.
In the production practice, because the depth of an oil well scales from a few kilometers to several kilometers, and the wellhead of the oil well is not likely to maintain dynamic coaxial with the polished rod. There are two reasons for this: first, static errors, that is, errors produced by non-coaxiality of the wellhead and the polished rod when the sucker-rod pumping machine is shut down, these errors include engineering errors brought about by underground and ground constructions, as well as cumulative errors due to geological changes, climate changes, and environmental changes over a period of time; and second, dynamic errors, that is, errors produced by non-coaxiality of the wellhead and the polished rod when the sucker-rod pumping machine is in the operating state, these errors include errors caused by jittering, bending, and the like of the polished rod when the sucker-rod pumping machine is in the operating state, and errors due to geological changes, climate changes, and environmental changes.
Since a position of the polished rod in the closing unit is changed in two states of ON and OFF, operating an existing declination rectification unit cannot realize static coaxiality between the packing and the polished rod, and the existing wellhead sealing device does not have a dynamic declination rectification function, thereby causing a result that the packing is always in an eccentric wear state due to dynamic non-coaxiality with the polished rod when the existing sucker-rod pumping machine operates (non-uniform wear of the packing by the polished rod is called eccentric wear).
The Chinese utility model patent with a publication number CN201024953Y discloses a multifunctional automatic declination-adjusting polished rod sealer, including a sealing device and an automatic declination-adjusting device, wherein the sealing device consists of a packing, a compression ring, a packing pressure cap and a handle; and the automatic declination-adjusting device consists of an adjusting ball, an adjusting pressure cap, a centralizing sleeve, an “O-shaped” ring and a main body. The utility model is improved on an original sealer and the adjusting ball is integrated with the adjusting pressure cap, the centralizing sleeve and the “O-shaped” ring into a whole in the automatic declination-adjusting device; an inner surface of the upper part of the main body is designed as a spherical shape, an inner surface of the middle part is designed as a conical shape, and an inner surface of the lower part is designed as a cylindrical shape. A combining part of the upper part and the middle part of the main body and a combining part of the middle part and the lower part of the main body are in dynamic matching with the polished rod, a sealing floating ball is mounted in the middle part of the conical shape; and a closing rubber valve is mounted at the lower part of the main body. The improved sealer may solve the problems of uneven wear of a sealing ring and loss of crude oil due to declination of an axis of the polished rod and an axis of a sealing main body, and may automatically adjust a declination; and when the polished rod is broken off, a well is automatically closed and the packing is conveniently replaced.
When the closing unit is in a closing state, it is necessary to ensure that the packing box and the polished rod are coaxial to facilitate the replacement of the packing and reduce the damage to the packing, the polished rod and an operator. However, a rubber valve type closing unit disclosed in the above patent and an existing rotary closing unit cannot be realized. When the sucker-rod pumping machine is shut down, adjusting the declination rectification unit must ensure that the packing box is coaxial with the polished rod. However, an existing flange ball type declination rectification unit and an existing eccentric ball type declination rectification unit are difficultly adjusted to an ideal state due to interference from the closing unit during on-site operation.
The Chinese utility model patent with a publication number of CN200985764Y discloses a wellhead blowout prevention device of a sucker-rod pumping machine, which consists of an oil cap, a dustproof ring, a pressure cap, a centralizing wheel train, a coupling sleeve, a spring, a conical spiral packing, a sealing ball head and a lower joint from the top down, wherein the centralizing wheel train is composed of upper and lower centralizing wheel trains, the upper centralizing wheel train is compacted at the upper part of the coupling sleeve by the oil cap, the dustproof ring and the pressure cap, and the lower centralizing wheel train is located at the bottom of the sealing ball head, the coupling sleeve is in threaded coupling with the sealing ball head, a ball head pressure cap is sleeved at the upper part of the ball head and in threaded coupling with the lower joint, the conical spiral packing is composed of upper and lower conical spiral packing groups, the upper conical spiral packing group is located within a conical body of the coupling sleeve, the lower spiral packing group is located at the upper part of a lower spiral packing base of an inner cavity of the sealing ball head, and the upper spiral packing group and the lower spiral packing group are compacted by an upper spring sleeve and a lower spring sleeve through the spring.
Because the sucker-rod pumping machine operates normally, a sealing ball and a baffle in the sealing ball type blowout prevention structure employed in this patent and an existing blowout prevention structure such as a baffle type blowout prevention structure are subject to different degrees of wear, when the polished rod is broken off, most of existing blowout prevention units do not play a role in blowout prevention.
Because the existing wellhead sealing devices have not completely solved the problems of eccentric wear and lubrication of the packing and the polished rod of the sucker-rod pumping machine, the existing wellhead sealing devices have the following defects: firstly, the service life of the packing and the polished rod is short, the difficulty in mounting or dismounting the packing is large; secondly, randomness in leakage and venting is high, the environment is polluted, workers are required to inspect wells every day and production and operation costs are high; and thirdly, the pressure and the friction force of the polished rod in most areas of the packing are too large, which results in excessive energy consumption in the pumping machine.